Electrorheological ("ER") fluids are fluids which exhibit substantial increases in viscosity in the presence of an electric field. Electrorheological fluids typically consist of a very low conductivity carrier fluid and a suspended particle component.
These fluids have previously been known by various names such as electrofluids, electroviscous fluids, electroresponsive fluids, electrorestrictive fluids, and jammy fluids. They are now most commonly referred to as "electrorheological" fluids because in addition to viscosity changes applied electrical fields can cause substantial changes in the overall rheology of the fluids.
The most commercially desirable ER fluids exhibit large, reversible electrorheological effects. In the absence of an electric field, these ER fluids exhibit Newtonian flow properties such that the shear stress (applied force per unit area) is directly proportional to the shear rate applied (relative velocity per unit thickness). When an electric field is applied, a yield stress phenomenon appears and no shearing takes place until the shear stress exceeds a minimum yield value which increases with increasing field strength, i.e. the fluid appears to behave like a Bingham plastic. This phenomenon appears as an increase in apparent viscosity of several orders of magnitude.
ER fluids change their viscosity characteristics very rapidly when electric fields are applied or released. The ability of ER fluids to respond this rapidly makes them highly desired elements in certain electro-mechanical devices. For example, ER fluids have applications in electro-mechanical clutches, fluid-filled engine mounts, coupling devices in robotic systems, automatic vehicle shock absorbers, and active dampers for vibration control. See e.g. U.S. Pat. No. 4,782,927. The disclosure of this patent, and of all other publications referred to herein, are incorporated by reference as if fully set forth herein.
For a discussion of the use of ER fluids in automotive devices, see generally D. L. Hartsock, et al. 35 J. Rheol. 1305-1326 (1991). For a general discussion of ER fluids, see R. Pool, 247 Science 1180-1181 (1990); U.S. Pat. No. 4,992,192; U.S. Pat. No. 5,032,307; and U.S. Pat. No. 5,032,308. For a discussion of the relaxation mechanism of ER fluids see D. Klingenberg, 37 J. Rheol. 199-214 (1993). See also U.S. Pat. Nos. 4,744,914 (zeolite particles); 4,994,198 (silicone particles); 4,502,973 (halogenated liquids); and 4,812,251 (fluorosilicone liquids).
Various problems have slowed the commercialization of ER fluids. One problem is the very high cost of certain components that are sometimes used to enhance the effect. Another problem is that for some applications there may be too small a viscosity response for a given electric field. For example, the automotive industry is interested in ER fluids for inter alia electrically controlled shock absorbers that continously stiffen and relax in response to changing road conditions. The electrical power used must be kept to the minimum (to avoid battery drain and heat dissipation problems), while the changes needed in viscosity to achieve the functions are great.